"All the 84 members of the Muslim Brotherhood were released today ... amid celebrations in front of the prison in Tripoli in the presence of their families," one source said on Thursday.
"Fivty-five of them have returned home to Benghazi," Libya's second city in the east of the country, added the source, who asked not to be named.
Libya arrested at the end of the 1990s 152 members of the Muslim Brotherhood.
In 2002 two were sentenced to death, 73 to life in prison and 66 were acquitted. The others were handed 10 year jail sentences.
The condemned, mainly students and academics, were accused of supporting or belonging to al-Jamaa al-Islamiya al-Libiya, an Islamist group created in 1979, whose beliefs reflect those of Egypt's banned but tolerated Muslim Brotherhood.